1. Field of the Invention
The present invention relates to a fiber Bragg grating (FBG) system, for use in an encoder or a decoder in an optical code division multiplex transmission system, and particularly to an FBG system that may control the Bragg reflection wavelength.
2. Description of the Background Art
The recent spread of the Internet or the like has rapidly increased communication demand. Accordingly, a high speed and large capacity network using an optical fiber or the like has started to be built. One communication means that has gained attention for constructing a high speed and large capacity optical network is an optical code division multiplex (OCDM) transmission.
In the OCDM transmission, plural channels of optical pulse signals are generated by optically modulating an optical pulse train to thereby convert electrical pulse signals into optical pulse signals and encoded with codes different from channel to channel, and the plural channels of optical pulse signals are decoded by a decoder on the receiver side into the original optical pulse signals with the same codes as used in the encoding on the transmitter side. Light for conveying optical pulse signals may sometimes be referred to as an optical carrier.
The OCDM transmission may convey optical pulse signals on a large number of channels simultaneously on the same wavelength. The OCDM transmission system uses one and the same code between the transmitter and receiver sides as a key, as which referred to may be a code that is set in the encoder and decoder, thus being higher in security for transmission as is one of the features.
One of the known encoding measures in OCDM is, for example, a phase coding OCDM, which uses an optical phase as a code. Specifically, the encoder and decoder use the super structured fiber Bragg grating (SSFBG). Another one of the known encoding measures in the OCDM system, in addition to one using the phase coding, includes a system using a wavelength hopping code. For any of the above encoding measures, a change in the Bragg reflection wavelength of the SSFBG system used in an encoder and a decoder caused by fluctuation in ambient temperature surrounding the SSFBG grating or the like has to be prevented, as will be described below.
The OCDM transmission uses, for any of the above encoding schemes, one and the same code as a key on the transmission and receiver sides. In the following discussion, for simplicity, the SSFBG may sometimes be abbreviated only as FBG.
For a better understanding of the present invention, a reference will now be made to schematic block diagrams of FIGS. 1 and 2 to describe the configuration of a general OCDM transmission system and an FBG grating for use in an encoder and a decoder in the OCDM transmission system. FIG. 1 schematically shows the configuration of an OCDM transmission system. FIG. 2 shows an FBG made of a 15-bit code where fifteen unit FBGs are arranged in the order of “ABCDEFGHIKLMNOP.” The FBG shown in FIG. 2 includes an optical fiber 6 that includes an SSFBG forming section 8 having the fifteen unit FBGs arranged serially in its waveguide direction.
The OCDM transmission system includes a transmitter section 10 and a receiver section 40, which are interconnected by a transmission line 42. Signals transmitted by the OCDM transmission system are of optical pulses. Optical pulse signals are binary digital signals that carry information to be transmitted. The optical pulse signals may, for example, be generated in the form of return-to-zero (RZ) type of optical pulse signals.
The transmitter section 10 includes an optical pulse train generator 12, a signal generator 14, an optical modulator 16, a first optical circulator 18, and an encoder 60. The optical pulse train generator 12 generates an optical pulse train 13. The signal generator 14 supplies the optical modulator 16 with information to be transmitted in the form of binary digital electrical pulse signal 15.
The optical modulator 16 outputs an optical pulse signal 17 to be transmitted, which is then input via the first optical circulator 18 to the encoder 60. The encoder 60 produces and sends an encoded optical pulse signal via the first optical circulator 18 in the form of optical pulse signal 19 to the transmission line 42, the optical pulse signal being in turn sent over the transmission line 42 to the receiver section 40.
The receiver section 40 includes a second optical circulator 22, a decoder 62, an optical coupler 26, an optoelectrical converter 28, a wavelength monitor 30, and a wavelength controller 32. The optoelectrical converter 28 converts an optical pulse signal 27 into a corresponding electrical pulse signal 36. The wavelength monitor 30 measures the degree of autocorrelation, or the amplitude of eye opening, of an optical pulse signal 29. The wavelength controller 32 receives an output 31 from the wavelength monitor 30 and supplies a control signal 67 to a temperature controller 68. The temperature controller 68, when received the control signal 67, is responsive to the control signal 67 to control, via a cable 69, the current of a thermo module 66 so as to regulate, i.e. increase or decrease, the FBG temperature.
The transmission line 42 conveys an optical pulse signal 21, which is in turn input via the second optical circulator 22 to the decoder 62 where the optical pulse signal 21 is decoded. The decoded optical pulse signal is sent back to the second optical circulator 22 and further to the optical coupler 26 where the optical pulse signal is split into optical pulse signals 27 and 29. The one optical pulse signal 27 is restored by the optoelectrical converter 28 into the corresponding electrical pulse signal 36. In this way, the binary digital electrical pulse signal 15 that carries information to be transmitted is received and restored by the receiver section 40 to the binary digital electrical pulse signal 36.
The decoder 62 has a temperature sensor 64 thereon that regularly measures the temperature of the FBG included in the decoder 62 and sends a measurement to the temperature controller 68 in the form of temperature signal 65. The wavelength controller 32 is responsive to the output 31 from the wavelength monitor 30 to calculate a temperature value to be set for the FBG. To achieve the calculated temperature, the temperature control signal 67 is supplied to the temperature controller 68.
Between the encoder 60 and the decoder 62, the FBGs are the same as each other in effective refractive index periodic structure but opposite to each other in periodic structure. Specifically, when the FBGs included in the encoder 60 and the decoder 62 have, as shown in FIG. 2, the fifteen unit FBGs arranged in the order of “ABCDEFGHIKLMNOP,” and the FBG included in the encoder 60 has its input/output port set on the side of the unit FBG denoted by “A”, the FBG included in the decoder 62 will have its input/output port set on the side of the unit FBG denoted by “P”.
The FBGs included in the encoder or decoder have the Bragg reflection wavelength, which may hereafter be referred to as “operating wavelength” and depends on the ambient temperature or the like. Suppose here that some cause such as an ambient temperature change gives rise to a difference in effective refractive index periodic structure of the FBGs between the encoder 60 and the decoder 62, thus providing the different operating wavelengths. Under that circumstance, it is necessary to control the FBG temperature so as to render the effective refractive index periodic structure of the FBG forming the effective refractive index periodic structure of the decoder 62 identical to the effective refractive index periodic structure of the FBG included in the encoder 60.
When installing FBGs in an encoder and a decoder, it is practically difficult to set the operating wavelength identical between the encoder and the decoder.
In order that the operating wavelength is always maintained identical between the FBGs included in an encoder on the transmitter side and an decoder on the receiver side, the operating wavelength of the FBG needs to be adjusted on at least either of the encoder and decoder.
With the phase coding OCDM transmission, when the FBG included in an encoder on the transmitter side has its operating wavelength that differs by several-ten picometer (pm) or more from the operating wavelength of the FBG included in a decoder on the receiver side, the receiver side may not decode successfully. This means that adjustment is always necessary on the wavelength of the FBGs included in the encoder on the transmitter side and the decoder on the receiver side so as to have the Bragg reflection wavelengths differ by less than several-ten picometer.
An FBG system has then been proposed which is so designed that the FBG may have its Bragg reflection wavelength hard to be affected by a change in ambient temperature, for example, by an international publication, WO 97/26572.
The international publication discloses a system that includes a negative thermal expansion substrate and an optical fiber provided on its surface at least two positions spaced apart from each other. The optical fiber includes an FBG. Now, with reference to FIG. 3, a description will be made of the dependency of the operating wavelength on a change in ambient temperature in the FBG system disclosed in the international publication. The x axis indicates the ambient temperature in degree centigrade (° C.) and the y axis the operating wavelength in nanometer (nm) of the FBG system. A straight line denoted by “a” corresponds to the FBG that is not fastened on the negative thermal expansion substrate. Another straight line denoted by “b” corresponds to the FBG that is fastened on the negative thermal expansion substrate and included in the FBG system. The operating wavelength of the FBG system refers to the peak wavelength of the Bragg reflection of the FBG forming the FBG system.
For the ambient temperature changing from 40 degree centigrade below zero to 125 degree centigrade above zero, the line denoted by “a” for an FBG that is not fastened on the negative thermal expansion substrate has its operating wavelength equal to 1563.75 nm at −40 degree centigrade, while 1565.65 nm at +125 degree centigrade, providing a difference of 1.9 nm therebetween. In contrast, the line denoted by “b” for the FBG that is fastened on the negative thermal expansion substrate and included in the FBG system has its operating wavelength variable in a range between 1565.5 nm and 1565.7 nm with a smaller difference equal to 0.2 nm, i.e. 200 pm. Specifically, an FBG fastened on the negative thermal expansion substrate and included in the FBG system may have its operating wavelength controlled with a change limited to 0.2 nm.
When the FBG system is used as an encoder and a decoder in the OCDM system, however, the change of 0.2 nm in operating wavelength due to an ambient temperature variation is too large to make the FBG system available as an encoder and a decoder. In the system disclosed in the international publication, once the FBG-incorporating optical fiber is fastened on an FBG system, the operating wavelength may not be controlled to any value by an external instruction. The OCDM optical communication involves a problem that when a light source for generating optical pulse signals to be sent from a transmitter side has fluctuation in wavelength or the like, it is hard to control the operating wavelength to compensate for the fluctuation.
For the purpose of solving the above problems, an FBG system has been developed which has a function of being responsive to an instruction from an external to control the temperature, when the FBG system for use in an encoder and a decoder has variations in its operating wavelength due to fluctuations in ambient temperature or when the light source for generating optical pulse signals to be sent from the transmitter side fluctuates in wavelength, to convert the operating wavelength to any wavelength with its adjustable range equal to 200 pm or more, and to finely adjust the operating wavelength with an accuracy of 1 pm, as is disclosed by, for example, Japanese patent laid-open publication No. 2005-173246.
The FBG system disclosed by the Japanese '246 publication features a control that causes little variation in operating wavelength even when the ambient temperature varies. It does however face the following problems when the FBG system is fabricated as an encoder and a decoder in the OCDM transmission system.
The OCDM transmission system is generally adapted to allow for one-way transmission, as shown in FIG. 1, from one terminal, transmitter section 10, to another terminal, receiver section 40, as well as both-way transmission. Each terminal thus includes transmitter and receiver sections, so that the encoder and decoder are disposed in close proximity to each other. It would therefore be expected that separate temperature control between the encoder and the decoder consumes more power. Specifically, when the terminal including the encoder and decoder in close proximity encounters an increase or decrease in ambient temperature, the encoder and decoder both experience the same control, and the temperature control in the encoder and decoder interact with each other, thus consuming more power in temperature control of the encoder and decoder.
For example, an abrupt increase in ambient temperature causes the temperature controller to decrease the temperature of the FBGs included in both encoder and decoder. Both encoder and decoder thus release more heat, requiring more power to be supplied to both temperature controllers. The same holds true for an abrupt drop in ambient temperature. Both encoder and decoder need to be supplied with power, thus also requiring more power to be supplied to both temperature controllers.
As described above, when the encoder and decoder are provided in close proximity to each other and thus release more heat, the heat releasing or absorbing sections of the encoder and decoder are in close proximity accordingly, hence causing a larger temperature increase of the heat releasing or absorbing sections than when they are disposed alone. Accordingly, the temperature controllers for the encoder and decoder may bear more burden. That is also the case with an abrupt drop in ambient temperature.